If you, like me, think that evdev and xinput should somehow help you to disable the touchpad without the need of some daemon or gui tool, you're right!

Having a look at the output of xinput list is already looks promising:

[18:11] kr:~% xinput list
⎡ Virtual core pointer                       id=2  [master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer                id=4  [slave  pointer  (2)]
⎜   ↳ Macintosh mouse button emulation          id=7  [slave  pointer  (2)]
⎜   ↳ TPPS/2 IBM TrackPoint                     id=8  [slave  pointer  (2)]
⎜   ↳ SynPS/2 Synaptics TouchPad                id=9  [slave  pointer  (2)]
⎣ Virtual core keyboard                      id=3  [master keyboard (2)]
    ↳ Virtual core XTEST keyboard               id=5  [slave  keyboard (3)]
    ↳ ThinkPad Extra Buttons                    id=6  [slave  keyboard (3)]
    ↳ AT Translated Set 2 keyboard              id=10 [slave  keyboard (3)]
    ↳ Integrated Camera                         id=11 [slave  keyboard (3)]
    ↳ Sleep Button                              id=12 [slave  keyboard (3)]
    ↳ Video Bus                                 id=13 [slave  keyboard (3)]
    ↳ Power Button                              id=14 [slave  keyboard (3)]
[18:11] kr:~% 

Having a deeper look at device 9, the touchpad, with xinput list-props 9 reveals an interesting setting:

[18:12] kr:~% xinput list-props 9 
Device 'SynPS/2 Synaptics TouchPad':
   Device Enabled (125):   0
   Device Accel Profile (244):   0
   Device Accel Constant Deceleration (245): 1.000000
   Device Accel Adaptive Deceleration (247): 1.000000
   Device Accel Velocity Scaling (248):   10.000000
   Evdev Reopen Attempts (242):  10
   Evdev Axis Inversion (249):   0, 0
   Evdev Axis Calibration (250): <no items>
   Evdev Axes Swap (251):  0
   Axis Labels (252):   "Abs X" (262), "Abs Y" (263), "Abs Pressure" (264), "Abs Tool Width" (265)
   Button Labels (253): "Button Left" (126), "Button Unknown" (243), "Button Right" (128), "Button Wheel Up" (129), "Button Wheel Down" (130)
   Evdev Middle Button Emulation (254):   2
   Evdev Middle Button Timeout (255):  50
   Evdev Wheel Emulation (256):  0
   Evdev Wheel Emulation Axes (257):   0, 0, 4, 5
   Evdev Wheel Emulation Inertia (258):   10
   Evdev Wheel Emulation Timeout (259):   200
   Evdev Wheel Emulation Button (260): 4
   Evdev Drag Lock Buttons (261):   0
[18:12] kr:~% 

If you expect something like xinput set-prop even without reading the manpage, you are right:

[18:12] kr:~% xinput set-prop 9 125 0

This disables the touchpad and can be integrated into .xinitrc very well!